The present invention is directed generally to the transmission of signals in optical communications systems. More particularly, the invention relates to systems, devices, and methods for dispersion compensation, for estimating and controlling the optical source frequency, and for estimating and controlling the optical source frequency in optical communication systems.
The development of digital technology provided the ability to store and process vast amounts of information. While this development greatly increased information processing capabilities, it was soon recognized that in order to make effective use of information resources it was necessary to interconnect and allow communication between information resources. Efficient access to information resources requires the continued development of information transmission systems to facilitate the sharing of information between resources. One effort to achieve higher transmission capacities has focused on the development of optical transmission systems. Optical transmission systems can provide high capacity, low cost, low error rate transmission of information over long distances.
The transmission of information over optical systems is typically performed by imparting the information in some manner onto an optical signal. In most optical transmission systems the information is imparted by using an electrical data stream either to directly modulate an optical source or to externally modulate an optical carrier so that the information is carried at the frequency of the optical carrier, or to modulate the information onto one or more subcarriers or sidebands, with the later technique sometimes called sub-carrier modulation (“SCM”).
Initially, modulated optical signals were spatially separated by placing each optical signal on a different fiber to provide space division multiplexing (“SDM”) of the information in optical systems. As the demand for capacity grew, increasing numbers of information data streams were spaced in time, or time division multiplexed (“TDM”), on the single optical signal in the SDM system as a means to better use the available bandwidth. The continued growth in demand has spawned the use of wavelength division multiplexing (“WDM”) to transport multiple optical signals on a single fiber. In WDM systems, further increases in transmission capacity can be achieved not only by increasing the transmission rate of the information on each wavelength, but also by increasing the number of wavelengths, or channels, in the system.
As the transmission rate of the data increase, chromatic and polarization mode dispersion (“PMD”) cause the signal to degrade thereby limiting data transmission rates and distances. One solution to the dispersion problem is to use dispersion compensating fiber. This increases the cost of the system and only allows for a fixed amount of compensation. If the dispersion changes over time, it is difficult to change the amount of dispersion compensating fiber to compensate for the change. PMD arises in optical systems when the different polarization modes propagate through the system with different group velocities. Dispersion compensating fiber does not independently compensate for PMD. Therefore, there remains a need to provide inexpensive and more flexible dispersion compensation in order to increase optical communication transmission rates and distances.
In addition, to further increase the data carrying capacity of optical communication systems, dense wavelength division multiplexing (“DWDM”) has been developed where multiple closely spaced wavelengths carry information. As the signal data rates increase, dispersion of the optical signal limits the transmission range before a signal must be converted to an electrical signal and regenerated as an optical signal.
A number of solutions have been used to overcome dispersion in DWDM systems. One solution is to use low dispersion fiber in the system. One problem with this approach is that low dispersion fiber has a low dispersion characteristic only in a limited range of wavelengths, so in systems transmitting a large number of channels, the dispersion is low for only a small number of wavelengths. Dispersion compensation fiber is expensive, and in many systems, existing fiber plants must be used. Another solution is to use dispersion compensating fiber (“DCF”), which applies a dispersion characteristic to the optical signal to counter the dispersion induced in the system. Again, DCF is expensive. Also, over time the dispersion characteristics of the system may change and this would require a changing the DCF used for compensation, which would be expensive and labor intensive. In addition to the problems described above, low dispersion fiber and DCF do not correct of polarization mode dispersion. One way to correct of for polarization mode dispersion is the carefully control the polarization of the optical signal, but the various methods for doing this are expensive and add complexity to the system.
As the wavelength spacing in DWDM systems decrease, it is necessary to more carefully control the transmit wavelength of the optical sources. Typically, an optical communication system will attempt to lock the optical source to the required wavelength by measuring the wavelength of the source and then adjusting the optical source to the correct wavelength. It is easier and quicker to lock optical sources with low phase noise and less temperature sensitivity, but these types of optical sources are expensive. Less expensive optical sources are available, but they have greater temperature sensitivity and phase noise making it more difficult to lock them to a specific wavelength using typical methods.
Because of the problems described above there remains a need to provide an optical communication system that effectively and inexpensively compensates for the effects of dispersion, including polarization mode dispersion. There is a further need to be able to compensate for the dispersion as the system changes over time without costly modification to the system. In addition, there remains a need for an optical communication system that uses less expensive optical sources that have greater wavelength drift and phase noise, but still allows for high speed DWDM data transmission.
Furthermore, in optical communication systems, optical sources are used by receivers to demodulate data carrying optical signals. In order to properly demodulate the data carrying optical signal, the receiver optical source frequency should be the same as the transmitter optical source frequency. This requires that the receiver optical source transmission frequency be carefully controlled to allow for proper demodulation and to avoid crosstalk and interference between adjacent wavelengths. Very stable and accurate optical sources are expensive, therefore it is desirable to use lower cost optical sources in receivers that may be controlled. One problem with lower cost optical sources is phase noise. Current methods of estimating optical source frequency and control take too much time to carry out and are corrupted by phase noise. Therefore, there remains a need to provide an accurate estimate of the receiver optical source frequency in the presence of phase noise.